In an image forming apparatus using electrophotography, image formation processes as follows are generally performed. Specifically, a surface of a photosensitive drum (image bearing member) is uniformly charged with a predetermined polarity and potential by a charging device. Then, an electrostatic latent image of an original image is formed by an exposure device. Next, the electrostatic latent image is developed by a developing device. As a result, a toner image is formed on the surface of the photosensitive drum. The toner image is transferred to a sheet in a transfer section. As an alternative method, the toner image is not directly transferred from the photosensitive drum to the sheet, but is transferred to the sheet through an intermediation of an intermediate transferring member as a transfer belt.
There exists a charging device using a roller-shaped charging member (charging roller) as the charging device that charges the surface of the photosensitive drum. The charging roller is provided to be in contract with the surface of the photosensitive drum or in proximity thereto. A predetermined voltage is applied to the charging roller to charge the surface of the photosensitive drum.
On the other hand, after the toner image is transferred to the sheet, an extremely small amount of toner or components thereof, paper powder, or the like remains on the surface of the photosensitive drum. The residual material electrostatic adhering to the surface of the photosensitive drum becomes an obstacle to a next image formation process, and hence the surface of the photosensitive drum is cleaned. However, there arises a problem of the physical adhesion of the residual material to the charging roller, which is caused by the imperfect cleaning of the surface of the photosensitive drum.
In particular, if the amount of toner components (external additive for a toner) physical adhering to the surface of the charging roller increases, a phenomenon that the surface of the charging roller is whitened (hereinafter, the phenomenon is referred to as “whitening”) occurs. When the surface of the charging roller is whitened, an electric resistance of the surface of the charging roller increases. As a result, the whitened portion of the surface of the charging roller becomes less likely to be charged. Moreover, a partial whitening of the surface of the charging roller causes nonuniformity in charging in a corresponding portion of the surface of the photosensitive drum. As described above, the physical adhesion of the external additive for the toner to the surface of the charging roller becomes an obstacle to uniform and suitable charging of the surface of the photosensitive drum. Therefore, the surface of the charging roller is required to be cleaned.
A brush roller as a cleaning device for the charging roller is well known. The brush roller includes brush filaments which clean the surface of the charging roller during contact. The brush filaments remove external additive of the toner adhering to the surface of the charged roller, as well as disperse the external additive of the toner. As a result, nonuniformity in charging of the surface of the photosensitive drum is inhibited. In order to clean the charged roller properly, the brush roller needs to contact the charging roller orthogonally at a predetermined pressure. Thus, if the rotation of the brush roller is stopped, a plastic deformation of the brush filaments is caused. The plastic deformation causes the brush filaments to flatten (hereinafter, this state is referred to as “flattening of the brush filaments”). If flattening of the brush filaments occurs, the cleaning performance for the charging roller is remarkably lowered. Therefore, it is difficult to maintain the cleaning performance of the brush roller over a long period of time.